Beat the butter in a small bowl with an electric mixer until as white as possible. Don't be too worried if it doesn't turn white, it's just as good yellow.

Beat in 1/2 of the sifted icing sugar and 1/2 of the milk until fluffy, then add the remaining milk and icing sugar until light and smooth.

Add food colouring (optional) and continue to beat until colour is evenly mixed.

Tips

When softening hard butter dont let it melt to the point were it's liquid. It ruins the whole icing.:
Add food colouring to make the butter cream a different colour or just leave it yellow.
Add 1 teaspoon cocoa powder for a chocolate tasting icing.